(CNN) -- Naomi Zikmund-Fisher says her son's life was saved by her maternal instincts -- and her son's flat ears.

Jesse's mastoiditis was discovered after his mother noticed that something was not right with his ear.

One Friday night in May, Zikmund-Fisher looked across the dinner table at 3-year-old Jesse and noticed that his right ear, which usually lies close to his head, was sticking out. She went over to check it out and found a big red lump behind the ear.

She e-mailed her pediatrician, who said that it was probably just a mosquito bite and that as long as Jesse otherwise felt fine, not to worry about it.

Jesse did otherwise feel fine, but something set off Zikmund-Fisher's "mommy meter." So the next day, even though nothing had changed, she called the pediatrician's office and spoke to the nurse.

Her husband took Jesse to the emergency room, and doctors determined that he had a classic case of mastoiditis: an infection of the mastoid bone, just behind the ear. If not treated with antibiotics, mastoiditis can be deadly.

Now Zikmund-Fisher tells other parents to trust their instincts and act, even when a pediatrician tells them not to worry.

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Dr. Assaad Sayah, chief of emergency medicine for the Cambridge Health Alliance in Massachusetts, agrees that parents should use their instincts when deciding whether their child needs quick medical attention.

"'When in doubt, bring them in' should be your first line of defense," he said. "If they don't look right to you, call your pediatrician, or take them to the emergency department, and if they look very sick, call 911." Watch for more tips on when to go to the ER »

Although relying on instinct is a good guideline, there are also some cut-and-dry situations when a parent really should to take a child to the ER. Here are five examples.

Any child 3 months old or younger who has a temperature over 100.4 F needs to be seen by a doctor, says Dr. Jennifer Shu, spokeswoman for the American Academy of Pediatrics. The fever can be a sign of infection or meningitis.

3. Head injury with loss of consciousness, confusion, headache or vomiting

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A burn should receive emergency care if it's larger than your child's palm, if it's deep or discolored, or if it was caused by a chemical, says Dr. David Beiser, who works in the emergency room at the University of Chicago Comer Children's Hospital. Burns on the hands, genitals and face are of particular concern, he says. Here's a quick primer on burns

5. Severe abdominal pain

Although many stomach aches are minor, others merit urgent attention. There are several signs to look for, including a swollen abdomen that's tender to the touch or blood in the stool, according to guidelines from Parenting.com and Dartmouth Medical School. Severe pain in the lower right side of the abdomen could be a sign of appendicitis.

After two shots of antibiotics, Jesse Zikmund-Fisher was back to his usual happy and active 3-year-old self. Even though it's been nearly a year since Jesse's visit to the ER, the lessons his mother learned are still fresh.

"If you are not satisfied with the answer you get, you need to push for the right answer," she said.